Electric discharge device



March 28, 1944. .1. H. HUTCHINGS ELECTRIC DI SCHARGE DEVICE .Filed 001:.25. 1941 F 0 .t n e V n I John H Hutchings,

His Attorney.

Patented Mar. 28, 1944 UNITED STATES? it to General Electric New YorkCompany; -a corporaticn otfl The presentdnvention relates toimprovementsin electric: discharge. devices, -more especiallydevicesawhich employ apool-typecathodesuch as a cathode consisting: of mercury;

. Mercury pooltubesare well adaptedfor'high currentuser because-oitherelatively unlimited electron emission :whichmay beobtainedfroma'mercur-y cathode, For this reason tubestof this .-type havefoundwideapplication in electricallycontrolled welding-systems and the like Where.very high-current loading. is involved, the-mercury pool, tube exhibitssome tendency to arc-back, aconditionwhich is a functionwofrthetemperature. ofthe tube or of I its internal parts.In-"order-to-mininrize this difliculty resort has been had towater-cooling-.- as an expedient. ior raising the safe operating limit.Thisapproach is entirely. satisfactory:as.-long-as,.relatively-:constant. loadsnare involved; on the. otherhand, since-water cooling; as'normally employed ine herentlyfunctions:to remove heat at a relatively uniform' ratav corresponding to theaverage rather than-to-the'maximum rate of energy dis. sipationeofrthe-cooled= device; the use -of such cooling does not automatically takecare Obtain-i porary load-. peaks. Accordingly,- in an-tapplica-v tioninwhich intermittent and;intense-- loading is-lcommon, as ls-truein-welding systems, 1'01 example, ,watez cooling-:is nota-completeanswer to the problemof arc-back".

The present invention. has as a principal ob ject the provision of animproved pool-tube-construction by which arc-back occurrences. due-tointcrmittentr load peaks may. be substantially avoided; In 1 thisconnection; an important. iea- V ture of-the invention consists in usingin..combinationn. withan. artificially cooled tube: a heat, absorbingjStIilCt-lll'fi? of sufficiently .large, thermal capacitmtos soak. up?relatively large instan= taneousdncrements: of heat.-. withoutpermitting. a i

structure-occupying the central region of the discharge: pathand-.afl'ording .a readily accessible collectingrsurfac for.' positiveions and mercury vapors .inolecn-lesa Theriaspectstoi-theinventionwhich-l desire to Protect: herein. are pointed.- out with-theparticularity alnithe; appended :claimso. I'he invention;.lt-. self,.togetherowith ,further. lobje'ctsi and ..-advan= tages: thereof may;;-bestbe understood by :reference to the following description takeninconnection with the accompanyingi drawing,.. .in

which-oblig 1.isa longitudinal sectional-.-view of a discharge devicesuitably. embodying the invention; land Fig-.,2 is-a.-transversesectionitaken on"line:2--2:of 1?ig..l.'.- v I I p Referring particula'rl.to. 1,."there is.i11u s.- trated a. .mercury. pool .tubeot. a typewhich .is particularly.) adapted lfor; use. in electronically controlled.weldingasystems, or in.any.;other.iap.. plication involvingintermittent loading. This includes; an elongated metalenveldpejhavingas onepart thereof a. cyllndenlo Twhichiis! constitutedoi anlappropriate structural; metal such as 10w carbon steel. Thecylinderg1 0 is closed. at its upper extremity by means .oiaflanged-header I I, andirom thisv there isosupporteda'an. anode l2which; extends; for a Substantial. distance along the axis of the.cylinder. 1 The. anode, which may, suitably consist; of. graphite, ismounted oniaheavy metal rod 13].an'di this, .in turn,.. is.irisulatinglmsupportze'd.from. the header ll through. a -glass..sleeve-l l.v The sle'eve,. is joined to thel. header through a metal. ring.vl5 and is .closed :at its luppenend. byimeans of a metal. cap ...I6Z,l.these .partsQbeingiormed. of a metal. such'. as, nickleironecobaltalloy which. is capablaof -.being-...se'aled.'directly to glass. Thecentral portion of thecap. lliis weldedior other.- wise..s01ld1y;-j0ined to the-upper. extremity 10f? the rod ...l 3, .and-a.pair .offfle'xible conductors.v H extendithrough an? opening in the.cap, into a socket in the end ofitherod; asolidjoint being pro, ducedbyl soldering,.,on.otherwise. I V The lower. end ioflthencyliriderl. lUjis closed. by a. second flanged header.-..-.20which is injcontact witha .poo1:-.type.1cathode comprising...a. body. of

mercury. 2 L. Theiheader reinforcedby .a.rela'- tively heavy ;disk-22.solidly. aiiixed tolits, bottom surface and .is .provided-.withlasterminal. stud... 24 for-conducting. current. to...the.. header 201 .andthence. to .the..ca.thode. 21". A tubulationi 25 whichnismwelde'd.-into;.the. header serves, as a means for. evacuating. the tube, duringits manufacturc. .f l

In order. to control the initiation. of a discharge, between v theanodel 2' and.;.the--v cathode 2 l there is provided. in. connectioniwith'the lat,- ter animmersion ..-starting...electrbde. 26.7 This compriseseibody or high resistance. material such as: boron .carbid'ela'ndisisupnorte'd in permanent. :contact with ithe cathode, {by means of ais positive. Accordingly the device may be made;

intermittently conducting so as to control the Pas a e of current to autilization device such'as a welding apparatus. Moreover, by appropriatephasing of the control potential applied to the electrode 23 the periodsof conductivity may be accurately regulated in such fashion as toproduce precisely regulated welding conditions;

order to prevent excessive spraying of cathode material into theinterelectrode space from the vicinity of the electrode" (that is, fromthe region where the cathode spotis'established) a baflie 32- isprovided in connection with the latter element.

In the application voil the device to a welding system the current whichthe device is required to carry is variable within widelimit's dependingupon the type of weld being made. For this reason, it is essential thatthede'viceshall be able to carry for brief intervals loads materiall inexcess of its capacity in continuous operation; This means that suchloads shall neither produce destructive heating of any of the parts ofthe device, nor result in defective operatlon.'- A particularconsequence of heavy loading which must be avoided is the occurrence ofarc-back, that is, a. discharge of electrons f romthe anode [2 to thecathode 2! during periods of inverse poem. tlal. The likelihood of,arc-back is in part a function of temperature conditionsexistingflwithin'the discharge space and tendsto increase as thetemperature is raised. Consequently, it is desirable to provide meansfor continuously cooling the walls of the discharge envelope, and thisis accomplished in the'present instance by the use of a cooling coil 35having its turns in heat exchangerelation with the cylinder I0: However,the rate of fluid flow through such a coil is, for reasons of economy,ordinarily fixed at a value. correlated to the'average rather than tothe peak rate of heat dissipation of the cooled device. Accordingly,while a cooling coil thus supplied functions satisfactorily as long'asthe loading of the device with. which it is associated does notdepart materially from the rated average value, it is nevertheless notreadily responsive to the rapid changesin condition which occur in adevice subjected to intermittently varying loads.

In order to avoid difliculties arising from" this latter circumstancalprovide'inaddition to the cooling coil 35 a mass of metal 40 01'relatively high thermal capacity and conductivity so disposed that it isalso in effectiveheat-exchanging relation with the internal'parts ofthe-discharge device. In the preferred construction illustrated in Fig.1, this is accomplished by disposing'the selected metal in an annularspace provided between the outer surface of the cylinder I0 and a largersteel cylinder which is arranged concentrically with it. In order tofacilitate assembly of the various elements, the cylinder 38 is joinedat one extremity to the cylinder 1 ID (i. e. as indicated at '39) sothat the annular space between the cylinders is "completely closed" atone end. This makes it possible first to assemb e e not only to fill thechamber between the cylinders l0 and 38, but also to cover the surfaceof theheader 20 toa considerable depth- Th isnitor electrode seal 30 maybe protected during this last operation by the provision of a protectivesleeve 4 l The metal 40 may be any one of the commonly available metalshaving good thermal capacity 15' and thermal conductivity materiallyabove that of iron, and in this connection one may use, for example,zinc, aluminum, brass or copper. Zinc is preferable in most instancesbecause of its low cost.

,With the construction specified the thermal capacity of the casingstructure of the discharge device is very much increased. Accordingly;in the event of a sudden increase in theloadin'gof the device, the heatgenerated may be'absorbed by the metal 40 and the associated structurewith only a very small rise in the temperature of the system as a Whole,and the likelihood-of arc-back due to excess vapor pressure brothercauses associated with increasing temperature will be minimized. Theheat soaked up'*by {the metal mass 40 during the emergency period-will,of course, be gradually removed by the circula tion of cooling fluidthrough the co'ils 35 in'the' ensuing period of normal operation (on-inwelding applications, of cessation of conduction) thus conditioning thedevice for the occurrence-ti another peak load. v

While the occurrence of arc-back is a function of temperature asstatedabove, it may be some what more directly attributable to ionsleft'in the discharge space in the period-immediately followingcessation of normal conductivity}; If such ions are present in quantitywhen iiiverse potential is supplied to the anode, it is known that thelikelihood of a-rc-backoccurrence is rela-' tively high, this beingapparently ares'ult ofthe tendency of such ions to collect at the anodesurface and to initiate emission from it. A further aspect of thepresent invention consists in'thc provision of means for shortening the'deionization time of a discharge device such as that shown inFig.1. Theparticular means which is illustrated for this purpose comprises aconductive structure extending for a substantial distance along thedischarge path between the cathode 2| and the anode l2 and occupying thecentral'region of the discharge space. In the illustrated arrangementthis structure consists of a hollow metal shell 43 which is welded intoa corresponding opening in the header 20 and which-forms, in effect, anupright colunm extending into close proximity to the lower surface ofthe anode l2 As a result of the presence of the shell 43 andin view ofthe fact that the-anode occupies a-maior portion of the cross-section ofthesp'ace enclosed by the cylinder l0 so that a straight linepath'exiists between the outer parts of its lower surface and the exposedportion of the cathode; the discharge path is of essentially annularform. It is, moreover,- relatively large areas so that there'is nonecessity for the discharge to be of contracted form with a consequentincrease in the voltage drop through the device. At the same time; thedistance from any point in thedischarge path to.

the nearest adjacent conductive and cooled surface is a small fractionof the radius of the discharge envelope, a condition quite differentfrom that which would prevail in the absence of the shell 43.Accordingly, any ions produced in the discharge space (i. e. between theanode and cathode) are reasonably certain to be collected within a briefinterval after generation, so that the possibility of substantialnumbers of residual ions being available to produce arc-back duringperiods of inverse potential is much reduced.

It is desirable that the temperature of the shell l3 be maintained at alow value during operation or the device and to this end a portion 35'of the cooling coil 35 may advantageously be disposed within the shell.

Eilective heat exchange between this part of the coil and the shell maybe assured by pouring into the shell a quantity 40 of the metal 40 whichis employed between the envelope cylinders l and 38, this metal beingappropriately introduced into both spaces at the same time. Obviously,the presence of the part 43 and the enclosed metal 40 still furtherincrease the thermal inertia of the structure and thus tends to preventany excessive temperature rise. With the arrangement described coolingfluid such as water may be circulated alon a continuous path from inlet46 through conduits 35 and 35' and finally removed through an outlet 41.

While the invention has been described by referenc to a particularembodiment thereof, it will be understood that numerous modificationsmay be made by those skilled in the art without departing from theinvention, I, therefore, aim in the appended claims to cover all suchequivalent variations as come within the true spirit and scope of theforegoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric discharge device comprising a relatively elongated metalcasing, a mercury pool occupying the lower portion of the casing, ananode supported directly above the cathode, a conductive structureextending tor a substantial distance along the discharge path anddisposed axially within the casing so as to occupy the central portionof the discharge space, and a mass of metal Of high thermal capacity andconductivity in thermal contact with the lateral walls of said casing,the heat-absorbin effect of said metal mass and the deionizing tendencyof said conductive structure servin conjointly to minimize thelikelihood of arc-back of said device upon the occurrence ofintermittent, intense loading.

' the cathode for a substantial distance along the discharge pathbetween the anode and cathode and disposed axially within the dischargeenvelope, a metal of relatively high thermal capacity and conductivityfilling said shell and occupying the said annular space between theenvelope cylinders, and a duct for cooling fluid extending through saidmetal for removing heat from said envelope and said shell.

3. A vapor discharge device comprising a cylindrical metal casing, amercury pool occupying the lower portion Of the casing, an anodesupported above the cathode and in cooperative relation with respect toit, and a conductive structure extending for a substantial distancealong the discharge path between the anode and cathode and disposedaxially within the casing so as to occupy the central portion of thedischarge space whereby the distance from any point in the dischargespace to a conductive surface is materially less than the radius of thesaid casing.

4. An electric discharge device comprisin a cylindrical metal casing, amercury pool occupying the lower extremity of the casing, an anodesupported above the cathode and occupying a major portion of the crosssectional area of the casing, and a metallic column extending from thecathode into relatively close proximity to the anode, said columnoccupying the central portion of the discharge space between the anodeand cathode and serving to facilitate rapid deionizetion of said space.

5. A vapor discharge device comprising an anode, a mercury pool cathodecooperating with the anode and a metal envelope enclosing the anode andcathode, said envelope comprising an inner steel cylinder directlysurrounding the discharge space, an outer steel cylinder of largerdiameter than the inner cylinder and joined thereto at one extremityonly so as to provide between the cylinders an annular space which isclosed at one end and open at the other, a cooling coil within saidannular space of such configuration as to permit its introduction intothe space through the said open end thereof, and a mass of metal of highthermal capacity and conductivity fused into said annular space incontact with said coil and with said inner cylinder.

JOHN H. HUTCHINGS.

